The following terms have the definitions as stated below.    Optic axis herein refers to the direction in which propagating light does not see birefringence.    A-plate and C-plate herein are the plates in which the optic axis is in the plane of the plate and perpendicular to the plate.    Polarizer herein refers to elements that polarize an electromagnetic wave.    In-plane phase retardation, Rin, of a film 201 shown in FIG. 1 is a quantity defined by (nx-ny)d, where nx and ny are indices of refraction in the direction of x and y, respectively and x-y plane is parallel to the plane 203 of the film d is a thickness of the film in z-direction. The quantity (nx-ny) is referred as in-plane birefringence. Both of these in-plane quantities will be treated as absolute values with no preferred sign convention.    Out of-plane phase retardation, Rth, of a film 201 shown in FIG. 1, herein, is a quantity defined by [nz−(nx+ny)/2]d. nz is the index of refraction in z-direction. The quantity [nz−(nx+ny)/2] is referred as out-of-plane birefringence, Δnth. If nz>(nx+ny)/2, Δnth is positive, thus the corresponding Rth is also positive. If nz<(nx+ny)/2, Δnth is negative and Rth is also negative.    Amorphous herein means a lack of long-range order. Thus an amorphous polymer does not show long-range order as measured by techniques such as X-ray diffraction.
Liquid crystals are widely used for electronic displays. In these display systems, a liquid crystal cell is typically situated between a pair of polarizers. An incident light polarized by the polarizer passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the second polarizer. By employing this principle, the transmission of light from an external source, including ambient light, can be controlled. The energy required to achieve this control is generally much less than required for the luminescent materials used in other display types such as cathode ray tubes (CRT). Accordingly, LCD technology is used for a number of electronic imaging devices, including but not limited to digital watches, calculators, portable computers, electronic games, and televisions for which light-weight, low-power consumption and long-operating life are important features.
Contrast, color reproduction, and stable gray scale intensities are desirable attributes for electronic displays, which employ LCD technology. The primary factor limiting the contrast of a LCD is the propensity of the light to “leak” through liquid crystal elements or cells, which are in the dark or “black” pixel state. Furthermore, this leakage and hence the contrast of a liquid crystal display are also dependent on the direction from which the display is viewed. Typically the optimum contrast is observed only within a narrow viewing angle range centered about the normal incidence to the display and falls off rapidly as viewing direction moves away from the display normal. In color LCDs, this leakage problem not only degrades the contrast but also causes color or hue shifts with an associated degradation of color reproduction. There are various modes of LCDs. Twisted Nematic (TN) LCDs are liquid crystal displays in which optic axis of liquid crystal rotates 90° in the azimuthal angle across the liquid crystal cell thickness direction when no field is applied. With a sufficiently large applied field, the liquid crystal optic axis becomes perpendicular to the liquid crystal cell plane except in the vicinity of cell bounding plate. In the vicinity of the cell bounding plate, the liquid crystal optic axis deviates from the cell normal direction. Vertically Aligned (VA) LCDs have liquid crystal optic axis that is substantially perpendicular to the liquid crystal cell plane without an applied field. This state corresponds to a dark state of the displays. With applied field, liquid crystal optic axis tilt away from the cell normal direction. Optically Compensated Bend (OCB) LCDs are liquid crystal displays based on the symmetric bow-shape bend state of liquid crystal optic axis. The bow-shape bend state occurs in the plane perpendicular to the liquid crystal cell plane and the state of bend is symmetric around the mid point in the cell thickness direction. In-plane switching (IPS) LCDs are liquid crystal display in which the field to change the direction of the liquid crystal optic axis is applied in the plane of the liquid crystal cell. In IPS LCDs, the liquid crystal optic axis changes its direction while remaining substantially in the plane of the liquid crystal cell.
LCDs are quickly replacing CRTs as monitors for desktop computers and other office or household appliances. It is also expected that the number of LCD television monitors with a larger screen size will sharply increase in the near future. However, unless problems of viewing angle dependence such as coloration, degradation in contrast, and an inversion of brightness are solved, the replacement of the traditional CRT by LCDs will be limited.
One of the common methods to improve LCD viewing angle characteristic is to use compensation films. Situated between a polarizer and a liquid crystal cell, a compensation film annuls the phase retardation imposed on the propagating light by the liquid crystal cell. Several LCD modes, with or without an applied field, exhibit positive C-plate symmetry that can be compensated by a compensation film with negative C-plate property. As is well known to those who are skilled in the art, TN and OCB Liquid Crystal Cell show C-plate symmetry in the center portion of the cell with sufficiently high, applied electric field. The VA Liquid Crystal Cell exhibits C-plate symmetry in the state without an applied field. This approximate positive C-plate state of a liquid crystal cell gives a dark state, if it is placed between the crossed polarizers. Here, crossed polarizers mean that transmission (or absorption) axes of two polarizers form an angle of 90±10°. The ray propagating perpendicular to the liquid crystal cell essentially does not see birefringence. That is the reason why in the normal viewing direction one has the highest contrast ratio in some modes of LCDs. On the other hand, obliquely propagating light rays see the phase retardation and this leads to light leakage in the dark state and degrades the contrast ratio. FIG. 2 schematically shows the principle of compensation. Ellipsoid 301 represents the positive C-plate that approximates liquid crystal cell 305 having nxlc=nylc=nolc, nclc=nelc where nelc>nolc. nelc and nolc are extraordinary and ordinary indices of liquid crystal. Compensation film 307 with negative Rth is shown by the ellipsoid 309. Here we have (nx+ny)/2>nz, thus giving a negative Rth. For the rays 311 obliquely propagating with an incident angle φ through the liquid crystal cell 305, the positive phase retardation from liquid crystal cell 305 is canceled by the negative phase retardation of the compensation film 307. Therefore, one can effectively prevent the light leakage caused by the birefringence of the liquid crystal cell in the oblique direction. Thus, compensation film for various modes of LCDs must have a negative Rth and simple methods to obtain this negative Rth are highly desirable for optical compensation of LCDs.
One of the essential attributes of LCD compensation is the wavelength dependence (or dispersion) of the phase retardation and birefringence versus the wavelength of light (λ). It is important to achieve a proper dark state without color shift. Phase retardation is directly proportional to the birefringence, and their dispersion shape (proper or reverse) is likewise related. “Proper” dispersion is such that the absolute value of phase retardation and birefringence increases toward shorter λ (i.e. towards the ultraviolet region). Conversely, materials with “reverse” dispersion have smaller absolute value of phase retardation and birefringence in the shorter λ region. Typical liquid crystal cells use materials with proper dispersion, which show larger positive values of birefringence and phase retardation toward shorter λ. For the negative birefringence case (the compensation film), proper dispersion gives larger negative value of birefringence and phase retardation at shorter λ. Conversely, negative birefringence with reverse dispersion (i.e. less desirable compensation films) exhibits less negative birefringence and phase retardation for light with shorter λ. The phase retardation dispersion of the liquid crystal cell and the compensation film have to be the same kind to properly cancel the phase retardation of a liquid crystal cell by compensation film. That is, if the liquid crystal phase retardation assumes larger positive value at shorter λ (i.e. positive birefringence with proper dispersion), the compensation films have to have larger negative value (i.e. negative birefringence with proper dispersion) of Rth (or Δnth) for good compensation.
FIG. 3 explains the concept. The liquid crystal cell in the positive C-plate state has a positive out-of-plane birefringence Δnlc=(nelc−nolc). The phase retardation to be compensated by a compensation film with negative out-of-plane birefringence and phase retardation is Rth-lc=dlcΔnlc (where dlc is a thickness of a liquid crystal cell). The curves show the wavelength dependence of Rth-lc=dlcΔnlc, 401 and Rth=dΔnth=d[nz−(nx+ny)/2] of the compensation films, 403, 407. A typical liquid crystal is positively birefringent and has proper birefringence dispersion. Namely, the birefringence Δnlc increases toward the shorter wavelength λ. Therefore, the phase retardation Rth-lc assumes a larger positive value at the shorter λ as shown by the curve 401. To counterbalance it, the compensation film is required to have also proper phase retardation dispersion shown by the curve 403. Films with the property shown by the curve 403 have a larger negative value of Δnth and Rth at the shorter λ. On the other hand, curve 407 shows the compensation film of reverse dispersion. In this case, the birefringence of the film Δnth and Rth has a smaller negative value at the shorter λ. Thus, proper cancellation of the phase retardation of a liquid crystal cell (a positive C-plate) cannot be achieved by a film with reverse dispersion for a wide range of wavelengths. Another important aspect of the compensation films is the transparency. At each wavelength λ, percent transmission of the film T(λ) can be measured. Percent transmission T(λ) gives the intensity percentile of the transmitted light with respect to the incident light in the film normal direction. The average transmission in the wavelength range λ1≦λ≦λ2 is defined as Tav=                    ∫                  λ          1                          λ          2                    ⁢              T        ⁡                  (          λ          )                                    λ        2            -              λ        1              .Optically transparent film is a film such that Tav≧90% for λ1=400 nm and λ2=700 nm. Such a film is preferred, as it does not compromise the brightness of the image on LCDs. Several means of obtaining negative Rth have been suggested. Sergan et al. discusses the crossed A-plate as a replacement of the negative C-plate (“Two Crossed A-plates as an Alternative to a Negative C-plate”, Society of Information Display 2000, pp. 838-841). Two A-plates are placed on top of each other with their optic axes forming 90°. They showed that crossed A-plates function approximately as a negative-C plate and successfully compensated the VA Liquid Crystal Cell. This method, however, involves cumbersome process of laminating two films with A-plate property to form one compensation film of negative-C behavior. Further, it is known to those who skilled in the art that crossed A-plates do not show negative C-plate behavior to rays with a small incident angle φ. Thus it is not a desirable method of obtaining negative Rth.
JP 1999-95208 discloses the use of a swellable inorganic clay layer in a crosslinked organic matrix to generate negative Rth. The disclosed method enables continuous means of manufacturing films with negative C-plate character. However, the resulting film gives the reverse dispersion, represented by a curve 407. Thus it is not suitable as a compensation film.
Li et al. (“Polyimide film as negative birefringent compensators for normally white twisted nematic liquid crystal displays”, Polymer, Volume 37, pp 5321-5325, (1996)) disclosed a polyimide layer formed by spin-casting or dip-emersion method. The film shows proper dispersion. This paper describes several polyimides that could be used as compensation films with negative Rth. However, the Tav is less than 90%. Also, as is well known to those who skilled in the art, polyimides generally suffer from a yellow-orange coloration. Thus, they are not desirable to be used as a compensation film because the color of the film would shift the hue of the images on the LCD's.
The use of a biaxially stretched cellulose ester film as a compensation film is disclosed in JP 2002-236216. Biaxially stretched cellulose ester film with large negative Rth and positive Rin is used as a substrate on which optically anisotropic layer with O-plate character is disposed. This biaxially stretched cellulose ester film (according to the disclosure) exhibits sufficient negative Rth to be useful as a compensation film. However, it possesses a reverse dispersion in Δnth, and thus in Rth.
Prior arts offer methods of obtaining films with sufficiently large negative Rth value. However, prior art films do not offer films with proper dispersion in negative Rth and Δnth that can be easily manufactured and have a high Tav(Tav≧90%, that is optically transparent). Also, it is highly desirable to obtain thinner compensation film with large negative Rth value. This enables overall LCD packages, comprising liquid crystal cell, compensation films, and polarizers to be slimmer. Thus, a desired method would offer a compensation film with sufficiently large negative Rth and the desired, proper retardation/birefringence dispersion, without significant increase in the display thickness.